The present invention relates to an amplifier for use in, for example, audio apparatuses, and more particularly to an amplifier which performs push-pull amplification on the input signal, thereby acquiring a high current efficiency.
FIG. 8 shows a conventional circuit disclosed in Jpn. Pat. Appln. KOKAI Publication 4-111507, which is designed for use in the output section of an audio apparatus. The circuit comprises two drive circuits A1 and A2, four transistors Q11 to Q14, two input terminals 71 and 72, a power-supply terminal 73, an output terminal 74, and a ground terminal 75. The inputs of the drive circuits A1 and A2 are connected to the input terminals 71 and 72, respectively. The transistors Q11 and Q13 constitute a first current mirror circuit, and the transistors Q12 and Q14 constitute a second current mirror circuit. The output of the drive circuit A1 is connected to the collector of the transistor Q11 and also to the bases of the transistors Q11 and Q13. The collector of the transistor Q13 is connected to the power-supply terminal 73. The emitter of the transistor Q13 is connected to the output terminal 74, together with the emitter of the transistor Q11. The output of the drive circuit A2 is connected to the collector of the transistor Q12 and also to the bases of the transistors Q12 and Q14. The collector of the transistor Q14 is connected to the output terminal 74. The emitter of the transistor Q14 is connected to the ground terminal 75, together with the emitter of the transistor Q12.
In the circuit shown in FIG. 8, an input signal A is supplied to an input terminal 71 through a capacitor C2, while an input signal /A, which is opposite to the signal A, in phase is supplied to the other input terminal 72 through a capacitor C3. The circuit operates as a push-pull amplifier. When its output terminal 74 is connected to a load RL through a capacitor C1, the circuit supplies the load RL with a current which is a product of the output current of the drive circuit A1 and the current mirror ratio of the first current mirror circuit or with a current which is a product of the output current of the drive circuit A2 and the current mirror ratio of the second current mirror circuit.
The drive circuit A1 and the transistors Q11 and Q13 constitute a first amplifier circuit, which amplifies the positive half of the input-signal wave. The drive circuit A2 and the transistors Q12 and Q14 constitute a second amplifier circuit, which amplifies the negative half of the input-signal wave. The first and second amplifier circuits are identical in structure. The circuit shown in FIG. 8 therefore has but a little distortion rate.
Both drive circuits A1 and A2 output an idling current each, even while they are receiving no input signals. Therefore, a current which is the product of the idling current and the current mirror ratio of the first current mirror circuit flows through the transistor Q13, and a current which is the product of rib the idling current and the current mirror ratio of the second current mirror circuit flows through the transistor Q14. Hence, the crossover distortion in the circuit of FIG. 8 is small.
To have a great amplification factor, either current mirror circuit needs to have a large current mirror ratio. If the current mirror circuits have their current mirror ratios increased, however, the idling currents flowing in the transistors Q13 and Q14 while no signals are being supplied to the drive circuits A1 and A2 will increase. Consequently, the circuit of FIG. 8 will consume more power.
FIG. 9 shows a B-class push-pull amplifier which is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-2009. In this amplifier, the transistors Q8 and Q10 remains on while no signals are being supplied to the input terminals 71 and 72. The emitter-current path between the diode-connected transistors Q7 and Q9 is rendered conducting. As long as this path remains conducting, the transistors Q7, Q8 and Q11 constitute a current mirror circuit, and the transistors Q9, Q10 and Q12 constitute a current mirror circuit. While no signals are being supplied to the input terminals 71 and 72, the idling current (i.e., output current) of either current mirror circuit is determined by the current mirror ratio.
While input signals are being supplied to the input terminals 71 and 72, the transistors Q8 and Q10 are alternately turned off in accordance with the polarities of the input signals. As a result, the transistors Q7 and Q9 are alternately turned off. When the transistor Q7 is off, all collector current of the transistor Q5 is supplied to the base of the transistor Q11. When the transistor Q9 is off, all collector current of the transistor Q3 is supplied to the base of the transistor Q12. The output current supplied from the output terminal 74 therefore depends on the current amplification factor of the transistor Q11 or Q12.
As indicated above, the transistors Q8 and Q10 remain on while no signals are being supplied to the input terminals 71 and 72. In these transistors Q8 and Q10, a collector-emitter saturation voltage VCE(SAT) is generated. The voltage VCE(SAT) causes the idling current to change by the value determined by the emitter-area ratio of the transistors Q7 and Q9 and the emitter-area ratio of the transistors Q11 and Q12. The voltages VCE(SAT) of the transistors Q8 and Q10 are difficult to control during the manufacture.
Furthermore, when the push-pull amplifier has its maximum current amplification factor, the transistors Q3 and Q4 or the transistors Q5 and Q6 are turned off. Since, the transistors Q8 and Q10 are saturated while operating, some times elapses until the transistors Q3 and Q4 or the transistors Q5 and Q6 are turned on. This inevitably increases the possibility of ringing.